1. Field of the Invention
The present invention relates to an adjustable sitting device, e.g. a chair, with the angle enclosed by the seat and the back rest being adjustable, if desired, with simultaneous adjustment of the level of said seat and back rest.
2. The Prior Art
Adjustment of the seat angle alone is of special importance for working chairs, and is necessary in situations requiring adjustment of the sitting level. Separate adjustment of the seat angle is common between an original position with the seat being inclined approximately 5.degree. forward and to a maximum of 15.degree. backward, i.e. a range of adjustment of approximately 20.degree.. A forward adjustment of the seat angle is also advantageous when the user is to rise from a chair, especially a chair for patients and users with impaired motory power. In this case it is, however, a condition that the sitting level may be adjusted while the user is seated on the chair.
Separate adjustment of the seat angle from 5.degree. forward and backward has no other function than that of reducing the angle of the user's body between the upper part of the body and the thighs.
The knee joint is the most important point of reference in case of angular adjustments. Provided that the chair is adapted for a special person the ratio between sitting level and sitting angle is commonly fixed. Adjustment of the seat angle should be stepless in the entire range of angles, and it should be possible to lock the chair at a desired angle. In connection with the above-mentioned, however, only adjustment of the seat was considered without taking the back rest into consideration.
In conformity with angular adjustment of the chair most of the weight of the body will act on the rear side of the turning point, but as opposed to an increased load in case of backward adjustment, the load will be reduced in case of forward adjustment. Balancing by spring suspended weights or the like will commonly be necessary.
Separate adjustment of the back rest angle is common in relation to a firmly fixed seat. Similar to adjustment of a seat With a back rest a comfortable position of rest will be achieved with approximately 30.degree. of backward tilting of the back rest. As opposed to adjustment of the seat with its back rest the angle of the body between the upper part of the body and the thighs will, additionally become more open.
Due to the fact that adjustment of the back rest occurs in relation to the seat the turning point should, ideally, be positioned in conformity with the hip joint of the user. The range of angular adjustment of the back rest should be from approximately 15.degree. backwards to at maximum a reclining position, if desired. An open angle of the body will have a positive effect on the breathing function and the circulation in the stomach region. A main disadvantage is, however, that the weight vector from the upper part of the body will cause the user of the chair to slide out of the seat, since the seat angle is not changed. If the turning point is, thus, not in conformity with the hip joint, displacement between the upper part of the body and the back rest will occur, however, dependent on the angle of the back. Due to this fact a possible neck rest and a support for the lumbar regions will change their positions relative to the upper part of the user's body.
By simultaneous adjustment of the seat and the back rest, so called sync adjustment there will be an established relationship between said portions. Commonly, the back rest will then move a double distance as compared with the seat. The great advantage of sync adjustment is that it maintains the main advantages of the above-mentioned methods of adjustment and eliminates the main disadvantages of them. Thus, it is possible to achieve a comfortable position of rest, slightly reclining backwards with an open angle of the body, and at the same time an inclined seat will prevent the user from sliding out of the chair. By individual adjustment a so-called asynchronous movement is achieved requiring two separate mechanisms with associated control members for the seat and the back rest, respectively. The main disadvantages of most existing chair arrangements is, thus, lack of cooperation between the chair and the adjusting mechanism and the fact that the mechanical structures are relatively conspicuous. Furthermore, considerations of anatomy are often neglected, and the operation of the adjustment members of the chair is cumbersome.
In connection with said adjustments, the adjustment of the seat level should also be mentioned. Often the back rest is adjusted at the same time as the seat. The level of the chair is adjusted to ensure maximum surface contact between the seat and the body of the user. A correct sitting position is achieved when the angle of the body equals the angle between the seat and the back rest of the chair, and the user's feet find good support on the floor, commonly at an angle equal to the angle of the back. The point of reference for adjustment of the level will, thus, be the knee joint.
Varying sitting levels are often required, depending on the working conditions or special requirements of the user. Any change of the seat level should always require adaption of the seat angle. The basic adjustment of the seat level must cover a range that is determined by the difference between the calf length of a big man and that of a small woman. Relevant data are found in antropometric tables. The range of adjustment for sitting level from the basic adjustment will also depend on the function of the chair and, form time to time, there is need for being able to adjust up to a half standing position.